K2-117
K2-117 (originally designated EPIC 211331236) is a rather large red dwarf star in the constellation of Cancer. The small star is around half the radius and almost 60% the mass of our Sun. In the early weeks of Exoplanet Explorers, citizen scientist Cabbink found a second transiting Super-Earth, making EPIC 211331236 one of the first stars to have more than one planet candidate around it found by the project. Several months later, in October 2017, both planets were confirmed by Dressing et al. Nomenclature and History K2-117 was first observed and cataloged by the Two-Micron All Sky Survey, or 2MASS for short. It was given the designation of 2MASS J08552535+1028087, based on its Right Ascension and Declination. The WISE infrared telescope also observed the star, giving it the second designation of WISE J085525.36+102809.0. Following initial observations by the Kepler telescope in its K2 mission, the star was designated EPIC 211331236 until the confirmation of a planetary system, when it was re-named K2-117. It is the 117th K2 star to have a confirmed planetary system. After the system's confirmation, Cabbink suggested that the second planet be named "Palnos", which roughly translates from Latin as "React". The first candidate was found by Martinez et al. and reported to the database ExoFOP on January 19th, 2017. This new candidate, dubbed EPIC 211331236.01, was about 50% larger than Earth and took just 1.2 days to orbit the star. About 4 months later in May of the same year, Exoplanet Explorers user Cabbink found a likely second candidate, while another user, Libmar96, stumbled across the poorly known first candidate. This was one of the first times that EE users found a star with a system of multiple candidate planets. In October 2017, Dressing et al. was able to confirm the planetary nature of the two candidates. Stellar Characteristics K2-117 is a high-end red dwarf star of spectral type M1.0V. It is currently on the main sequence, and will remain so for billions of years. K2-117 is not only a rather bright red dwarf, it is also one of the larger ones. Its mass, radius, and temperature are very well constrained. The star is 0.491598 times the radius and 0.582006 times the mass of the Sun, with a temperature of 3,842.118652 degrees Kelvin. Based off of this, the calculated luminosity for K2-117 is 0.0471779 Lsun, or just under 5% the luminosity of the Sun. Currently, the star's distance from Earth is unknown, but it does have a visual magnitude of 14.655, meaning that one would need a telescope of at least 10" to see K2-117. Planetary System Currently, K2-117 is known to host two transiting planets. Both are significantly larger than Earth and are around the borderline between a rocky Super-Earth and a gaseous Mini-Neptune. The masses, and therefore the compositions, of these planets are not known. However, due to their close proximity to one another, and the rather large eccentricities of their orbits, it is likely that K2-117b and K2-117c receive large enough Transit-Timing Variations to determine their masses. K2-117b and c are very close in size to each other, with their radii differing by only about 320 km (0.05 Re). Both planets likely formed farther out from their host star, but eventually migrated close to the star together. This is supported by their close orbits, moderate orbital eccentricities, and rather large orbital inclinations. It also suggests that both worlds would have formed with a large quantity of volatiles, mainly water with decent amounts of hydrogen and helium. However, the planets' close proximity to the star, especially with K2-117b, suggests that both worlds have suffered extreme atmospheric loss. It's possible that K2-117b has lost most of its volatiles, while the more distant K2-117c may have retained more of its original gases and water. The two planets are near a 4:1 mean-motion resonance. In mid-October 2017, Hirano et al. also confirmed the planetary nature of the two planets and attempted to look for Transit Timing Variations. No such ones were found, suggesting that, unlike previous theories, K2-117b and c have retained much of their volatile compounds. Gallery Category:Red dwarfs Category:Red dwarf systems Category:Confirmed systems